The present invention relates generally to a method and apparatus for processing digital images, and in particular, to a method and apparatus for enhancing digital images from documents containing background scenes or other spurious images which tend to obscure or confuse the proper discernment of the desired images.
In document processing systems, it is desirable to enhance certain information on the document images which generally consists of hand- or machine-printed characters. As characters are made of thin lines, a filter which enhances these lines over background scenes or other spurious images, improves the quality of the document images and assists in discerning the content of the images for proper storing and processing by computer.
U.S. Pat. No. 3,973,239 issued Aug. 3, 1976 to Kakumoto et al discloses a pattern processing system wherein portions of an image detected by a pickup tube is stored in a matrix. A brute force type of multiplication is performed wherein a separate multiplier is used to multiply each element of the data containing matrix by a single element of a filter matrix, and the results summed by an adder. Thus an m.times.n matrix requires m.times.n multipliers. Also, the apparatus disclosed therein contains shift registers which shift downwardly in sequence until the value being shifted downwardly reaches the bottom of the data window, and thereafter the value is moved to the top of the immediately right column. This procedure is followed for all of the columns until the data matrix is filled.
U.S. Pat. No. 3,889,234 issued June 10, 1975 to Makihara et al discloses a feature extractor in which a series of bits representing an image is simultaneously shifted in a right shift register. In the disclosed device, all of the data is first set up in registers and then addressed.
Other examples of character enhancement and pattern discerning apparatus are disclosed in the following: U.S. Pat. No. 3,737,855 issued June 5, 1973 to Cutaia; U.S. Pat. No. 3,805,239 issued Apr. 16, 1974 to Watanabe; U.S. Pat. No. 4,047,152 issued Sept. 6, 1977 to Giuliano et al; U.S. Pat. No. 4,052,699 issued Oct. 4, 1977 to Micka et al; U.S. Pat. No. 4,162,481 issued July 24, 1979 to DuVall; and U.S. Pat. No. 4,266,249 issued May 5, 1981 to Chai et al.
Many sensors used with optical detection systems severely attenuate higher frequencies. Hence, most of the image energy at higher frequencies is due to noise introduced by the imaging system. When documents are scanned vertically, low frequency noise in the vertical direction appears as very high frequency noise in the horizontal direction. It is therefore desirable that higher frequencies, especially in the horizontal direction, be attenuated by a filter. This filter should also be capable of enhancing lines that are approximately two pixels thick. It has been found that black and white line pairs of two pixels thick are one dimensional spatial square waves with the fundamental frequency equal to F.sub.s /4, where F.sub.s is the sampling frequency of the image. Hence, the filter should amplify frequencies approximately equal to F.sub.s /4. It has also been found that the highest frequencies in the image sensed by scanning in a vertical direction correspond to F.sub.s /2. Hence, sinusoids with the approximate frequency of F.sub.s /2 should also be attenuated.